A breathing apparatus with a breathing mask for the supply of breathing gas during operations underground or in an atmosphere containing hazardous materials and with a display unit for measured data within the breathing mask has become known from DE-C 36 22 147. The measured signals that are supplied via a signal line, such as, e.g., the cylinder pressure of carried pressurized cylinders, are analyzed and evaluated in a signal processing unit and reported to the person wearing the unit via a display field of the display unit within the breathing mask. The display field usually consists of LED (light emitting diode) elements, which are fed from an energy source located in the signal processing unit. In this case, it is disadvantageous that a major part of the energy reserves stored in the energy source is exhausted by the operation of the LED elements. To make matters worse, during operations in sunlight, the luminous power of the LED elements must be increased by increasing the feed current, so that the display field can still be read satisfactorily, whereby, however, the energy reserves decrease even more rapidly. In contrast, during operations underground, low luminous intensities of the LED elements are sufficient to be able to still read the display field satisfactorily.
A photoelectric sensor with a light source with a receiver aligned with the light source and with a light-focusing element in front of the receiver has become known from DE-C 33 17 057. A fluorescent, monochromatic plastic having light-collecting properties (LISA plastic), which receives the light radiated by the light source via a light entering surface and which conveys the collected light to the receiver via a light exiting surface, is used as a light-focusing element. This plastic absorbs direct or diffuse light, which it absorbs via its light entering surface, from the environment. This light is emitted as fluorescent radiation in the plastic matrix, as it is conveyed to the light exiting surface by total reflection and released there. Light with wavelengths greater than 370 nm is suitable for such a light conversion.